strpos error

Could someone tell me why this aint working? I understand this could be done using strstr() function. I just gave it a try and I cant quite figure out why this piece of code
returns NULL though the search string is present in the main string.

Edit: Oops! This only works if t is at the end of s. This works; tested with s="abcjobxyz" and t="job"

Code:

while (*pb && (*pa == *pb))
{
if(pb == (b + strlen(b) -1))
return a;

Edit2: SaswatPadhi's right, you ALWAYS need to be suspicious of a single = in an expression of any type. = and == are NOT the same. if (a=b) is valid; it assigns b to a then evaluates TRUE or FALSE depending on the value of both, so it's useful in stuff like while (x=somefunc()) where somefunc returns zero when it's finished.